1. field of the Invention
The present invention relates to a temperature control device for controlling temperature of a heat roller used in an image forming device, and more specifically to a temperature control device which controls temperature of a heat roller by controlling ON/OFF of a heat generating sheet of the heat roller, wherein the heat generating sheet is attached on an inner surface or an outer surface of a cylindrical roller body.
2. Description of the Related Art
An image forming device, such as an ink jet printer and a laser printer, includes a heat roller for thermally fixing toner images or ink images onto a recording medium, such as a recording sheet or a film. A conventional heat roller includes a roller body and a halogen lamp. The halogen lamp is disposed inside of the roller body, separated from the inner surface of the roller body by an air space. The halogen lamp generates heat when supplied with an electric current. The heat radiates across the air space to the roller body, so that the temperature of the heat roller increases. A thermistor is disposed on the outer surface of the roller body. A CPU controls the halogen lamp, and consequently the temperature of the roller body, based on temperature detected by the thermistor. The air space between the heat roller and the halogen lamp provides excellent electrical insulation, so the thermistor is provided on the low voltage side of a transformer, that is, on the same side as the CPU. Therefore, the thermistor and the CPU are directly connected by an electrically conductive line. However, because the halogen lamp is usually provided near a rotational axis of the heat roller, its heat conductivity is inefficient, so surface temperature of the heat roller takes long time to reach a predetermined temperature from when supply of electric current is started.
Japanese Patent-Application Publication (Kokai) Nos. HEI-8-110723 and HEI-8-194401 disclose a heat roller with increased heat conductive efficiency so that the surface temperature of the heat roller can be quickly increased. The heat roller includes cylindrical roller body and a heat generating sheet. The heat generating sheet is formed from an insulation sheet provided with a resistor pattern, and is attached along the inner surface of the cylindrical roller body. The insulation sheet electrically insulates the resistor pattern from the roller body. Because the resistor pattern is disposed next to the inner surface of the roller body, heat generated by the resistor pattern is efficiently conducted and quickly increases the temperature of the heat roller. It should be noted that the heat-generating-sheet can be attached along an outer surface of the cylindrical roller body. In this case also, the heat conductivity, and consequently the speed at which temperature of the heat roller increase, can be improved.
It is conceivable to control for a CPU to control the surface temperature of the heat roller, to maintain the surface temperature to a predetermined temperature, based on surface temperature detected by, for example, a thermistor. However, in the above configuration, because no insulation air space is provided between the roller body and the heat generating sheet, the roller body is not reliably electrically insulated from the heat generating sheet. For this reason, the thermistor needs to be provided on the high-voltage side, that is, the primary side, of a transformer, that is, on the same side as the heat generating sheet. Because the CPU is provided at a low-voltage side, that is, the secondary side, of the transformer along with an operation panel and the like, the CPU cannot be connected to the thermistor by an electrically conductive line. Otherwise, there is a danger that a user may get an electric shock. Therefore, it is difficult to use a CPU to detect and control the temperature of a heat roller with a heat generating sheet.